A solder leveller is apparatus for tinning exposed metal--normally copper--on printed circuit boards, and similar elements--particularly for supporting electronic components--hereinafter referred to simply as "boards", with solder preparatory to connection of components thereto. Generally a solder leveller comprises a bath of molten solder into which a board is lowered vertically for tinning with solder and means for leveling solder deposited on the board as it si withdrawn from the bath. Such a leveller is the subject of British Patent No. 2151528 and equivalent U.S. Pat. No. 4,599,966, both in the name of the present applicant.
Such conventional solder levellers operate on a one-at-a-time basis, inserting boards into a deep, narrow soilder bath one-at-a-time from above. This is an essentially slow process, although the speed of withdrawal from the solder is relatively high in terms of speed of travel of boards along conveyors through pre- and post- treatment stages, up-and down-stream of the leveller. A feature of a vertical acting solder leveller is that the lower part of the board remains in the solder for longer than the upper part. This can result in formation of a thicker copper/tin intermetallic compound layer on the lower part to which it can be difficult to satisfactorily solder components. Such a thicker layer can cause a board to "age" more rapidly, aggravating solder-ability problems.
Intermetallic compounds can be both of the Cu.sub.6 Sn.sub.5 type and of the Cu.sub.3 Sn type. The latter is particularly problematic and more prone to formation where the board experiences longer dwell times in hotter solder.
Equipment for continuous soldering treatment is known in various forms, in particular for soldering components to pre-tinned boards. "Drag soldering" involves the dragging of a board with its components across the surface of a bath of molten solder, see for instance U.S. Pat. Specification No. 4,284,225 (EMP Ag). "Wave Soldering" involves passing a board across the top of a wave created, in a bath of molten solder, see for instance British Patent Specification No. 2,117,690A. (Zevatron GmbH).
Both drag and wave soldering have been adapted for continuous tinning. U.S. Pat. Specification No. 4,277,518 (Gyrex Corp.) describes the use of a continuous conveyor forced down into molten solder by a fixed upper bed to drag a board through the solder. The board tends to float up against the conveyor which causes frictional engagement of the board with the conveyor. The speed of forward operation is limited by potential slippage between the board and the conveyor.
U.S. Pat. Specification No. 4,465,014 (Siemens Ag) describes movement of a board through a solder wave so that solder wells on to the top surface of the board. Speed of operation is limited by time available for heat transfer from the solder to the board.
The object of the present invention is to provide an improved solder leveller capable of operating fast enough for effective operation of the levelling means and yet able to heat the board quickly in a compact solder bath with a short dwell time in the bath to minimize the formation of an intermetallic compound layer.
Another object of the invention is to provide a solder leveller including means by which a board is rapidly heated to the temperature at which exposed copper on the board is wetted by solder.
Another object of the invention is to provide a solder leveller operating substantially horizontally to provide even treatment of the entire board and in which the board is moved fast enough for successful air knife levelling.